Wait, so for giving blood do you just get back stage passes, or a free ticket?
I couldn't care less about a backstage pass, I just want to get in for free.
Donating blood gets you a wristband that allows you to be able to watch bands from onstage (all except for the mainstage) and gets you free water refills all day as long as you provide the empty water bottle. It was an absolute lifesaver when I went to Kansas City Warped back in 2010. If these aren't the same perks anymore than sorry for the misinformation!
edit: and no, you do not get a free ticket to Warped just for donating. I think the only way you get a free ticket is for volunteering, in which case you literally have to stay at the tent the whole day and work (or go out and pass out flyers or something). There won't be any time to watch any bands unless there is a stage close enough to your tent.